Wheel alignment machines are used by automobile manufacturers and service centers to determine and set various alignment characteristics of the vehicle wheels in accordance with the manufacturer's recommended specifications. These specifications are selected to meet the requirements of a given vehicle model and may therefore vary from one model to the next. Initially, the vehicle wheels are aligned by the manufacturer in accordance with the design specifications. Thereafter, the vehicle wheels should be periodically checked and realigned over the useful life of the vehicle. Such continued compliance with the manufacturer's recommended specifications is important in insuring proper road handling and minimizing tire wear.
For steerable wheels (e.g., the front two vehicle wheels), there are several wheel alignment characteristics that affect steering performance and tire wear. These characteristics include camber, caster, toe, and steering axis inclination (SAI). Two of these, caster and SAI angle, relate to the inclination of the steering axis from a purely vertical line and may be determined in various ways known to those skilled in the art. See, for example, U.S. Pat. No. 5,291,660, issued Mar. 8, 1994 to A. Koerner. The other two of these, camber and toe, relate to the orientation of the rotational plane of the vehicle wheel and are important to both steerable and unsteerable wheels. In particular, the camber of a vehicle wheel is a measurement of the inward or outward tilt of the wheel relative to a vertical plane extending in the vehicle's longitudinal direction. The toe of a vehicle wheel is a measurement of the inward or outward tilt of the wheel relative to a horizontal plane extending through the wheel's center.
Many techniques have been developed to determine the rotational plane and, hence, the camber and toe of a vehicle wheel. The vast majority of these techniques have required some type of physical contact with the vehicle wheel. For example, in some wheel alignment machines, position encoders are used to generate signals indicative of the positions of rollers that contact the tire's sidewall. Generally, three such rollers and associated encoders are used for each wheel, with the rollers being spaced ninety degrees apart. The encoders provide a data stream indicating the distances of the rollers from a vertically disposed plane that extends in the vehicle's longitudinal direction. This data is used by a central computer to calculate the camber and toe angles.
More recently, optical techniques have been developed that permit determination of these alignment characteristics without any targets or other parts of the measurement apparatus having to come into physical contact with the vehicle wheel. See, for example, U.S. Pat. No. 4,745,469, issued May 17, 1988 to T. J. Waldecker et al.; U.S. Pat. No. 4,863,266, issued Sep. 5, 1989 to S. Masuko et al.; and U.S. Pat. No. 5,268,731, issued Dec. 7, 1993 to M. Fuchiwaki et al. Non-contact optical measurement of wheel alignment characteristics is advantageous because it provides good resolution and the ability to determine alignment characteristics without moving parts and without requiring contact between the measurement equipment and the vehicle. For each wheel, actual measurement of the wheel position is accomplished using an optical sensor that includes a light source for projecting light onto the wheel and a light responsive receiver for sensing a portion of the projected light reflected off the wheel.
In the patent to Waldecker et al., wheel alignment measurement is accomplished using a non-contact sensor station located adjacent each of the vehicle wheels. The sensor station for each wheel utilizes a plurality of sensor modules, each of which includes a laser light source and an associated video camera. The laser light sources at each wheel are used to project stripes of laser light that extend radially across the sidewall at two or more spaced locations. The video cameras at each sensor station are used to sense reflections of their associated laser's light off the tire's sidewall. Each camera is offset from the optical plane of its associated laser to give it a perspective view that permits the determination of distance between the sensor and tire sidewall. Because of the curved contour of the tire's sidewall, the reflected line of laser light, as seen from the perspective of the associated video camera, will have a generally parabolic shape. The images sensed by the video cameras are provided to a computer system that includes an alignment processor engine and an integrated host/alignment processor engine, both of which employ separate VME buses for communication. These processor engines are used to analyze the images to determine the locations within the images of the reflected laser lines. This information is then used to derive the camber, toe, or both. The system can include a transition detection circuit that monitors the digital video stream as it is being written into memory to determine the crown of the reflected parabolic laser line, which represents the point on the tire sidewall that is closest to the sensor module. This is accomplished by turning each camera on its side such that the reflected laser line, as seen by the camera, has a generally vertical orientation. Each scan line of the camera will then intersect the reflected laser line and the closest point of the tire sidewall can then be found by determining the row (scan line) number and column number of the crown of the reflected parabolic laser line. This information can then be used by the processors to quickly determine the probable location of the closest point to the sensor module.
This prior art system suffers primarily from its complexity and the resulting cost to implement. More specifically, in the preferred embodiment, the system utilizes three lasers and three video cameras for each wheel to determine the camber and toe angles. For a vehicle having four wheels, this means twelve lasers and twelve video cameras are required. Additionally, the computer system acquires complete frames of data from each of these cameras and must then utilize the two VME based processors operating in parallel to perform image processing to determine the location within each frame of the reflected laser light. Furthermore, the sensor stations do not themselves provide data indicative of the distance between the tire and sensor; rather, they only provide video frames and, therefore, their output data cannot be used with existing wheel alignment computers or in a conventional manner to determine the wheel alignment characteristics.